PYB304 Study

Allele

One of two or more alternate versions of a gene that occupy the same position on chromosomes. They control the same trait but may produce different outcomes (e.g. brown vs white seed colour).

Homozygous

Having two identical alleles for a given gene (e.g. BB or ww). Can be either dominant (BB) or recessive (ww).

Heterozygous

Having two different alleles for a given gene (e.g. Bw). The dominant allele will typically be expressed in the phenotype.

Genotype

The actual genetic make-up of an organism - the specific alleles it carries; what is in the DNA.

Phenotype

The observable characteristics of an organism that result from the interaction of its genotype and environment (e.g. eye colour, height, behaviour).

Dominant

An allele that is expressed in the phenotype when present in either the homozygous or heterozygous condition. Only one copy is needed for expression.

Recessive

An allele that is only expressed in the phenotype when present in the homozygous condition (two copies needed). Masked when paired with a dominant allele.

The Central Dogma

The fundamental process of gene expression: DNA → (transcription) → RNA → (translation) → Protein → Phenotype. Describes how genetic information flows from gene to behaviour.

Transciption

The first step of gene expression. A segment of DNA is used as a template to synthesise messenger RNA (mRNA). Occurs in the cell nucleus.

Translation

The second step of gene expression. Ribosomes read mRNA in 3-base sequences (codons) to assemble chains of amino acids that fold into proteins.

Polygenic

When multiple genes each contribute to a single trait (e.g. height, intelligence, depression). Most complex traits are polygenic.

Pleiotropic

When a single gene influences multiple traits or characteristics.

Epigenetics

Changes in gene expression that don't alter the underlying DNA sequence, but are caused by environmental factors. Main mechanism is DNA methylation.

DNA Methylation

The addition of methyl groups to DNA, which typically silences gene expression. The primary epigenetic mechanism. Can be triggered by environmental factors (stress, diet, trauma).

Monozygotic (MZ) twins

Identical twins. Develop from a single fertilised egg; share 100% of their DNA.

Dizygotic (DZ) twins

Fraternal twins. Develop from two separate fertilised eggs; share ~50% of DNA (same as any siblings). Born at the same time but genetically no more similar than siblings.

MAOA Gene (‘warrior gene’)

A gene encoding enzymes that break down neurotransmitters. The risk allele is associated with aggression only in combination with childhood abuse, an example of gene effect being environmentally dependent.

Meiosis

Cell division that produces gametes (sperm and egg cells). Each gamete contains only half the full chromosome set (23 chromosomes in humans). Enables genetic recombination.

Phenylketonuria (PKU)

A genetic disorder caused by a single recessive gene. Individuals lack phenylalanine hydroxylase (an enzyme), causing toxic build-up of phenylalanine. Causes intellectual disability and seizures, but symptoms can be prevented with dietary restriction.

Central nervous system

Brain + spinal cord — located within the skull and spine. Uses nuclei (cell body clusters) and tracts (axon bundles).

Peripheral nervous system

Everything outside the skull and spine - carries signals to and from the CNS. Uses ganglia and nerves.

Afferent

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Signals travelling towards the CNS (sensory input). Dorsal spinal roots.

Efferent

Signals travelling away from the CNS (motor output). Ventral spinal roots.

Sympathetic nervous system

Arousal and fight-or-flight responses; projects from lumbar and thoracic spinal cord.

Parasympathetic nervous system

Relaxation and rest-and-digest; projects from brain or sacral spinal cord. Every organ receives both opposing inputs.

Meninges (3 layers)

Three protective membranes surrounding the brain and spinal cord. Dura mater: tough outermost layer. Arachnoid meninx: spider-like middle layer. Pia mater: delicate innermost layer that directly adheres to the cortex surface.

Blood-brain barrier

A selective barrier formed by tightly packed endothelial cells lining cerebral blood vessels. Only small molecules, lipid-soluble molecules, and some gases pass freely. Large molecules like glucose require transporter proteins. Prevents most toxins from reaching the brain.

Myelencephalon (medulla)

The most caudal (lowest/most primitive) brain division. Contains the medulla oblongata and reticular formation. Functions: carries signals between brain and body; controls respiratory, cardiac, and circulatory reflexes. Life-critical.

Metencephalon

Contains the pons and cerebellum. Pons ('bridge'): bulge-like structure containing the reticular formation — involved in sleep, arousal, and relay of signals. Cerebellum ('little brain'): essential for sensorimotor coordination and balance; may have broader cognitive functions.

Mesencephalon (midbrain)

Contains tectum and tegmentum. Tectum: superior colliculi (visual-motor responses) and inferior colliculi (auditory). Tegmentum: periaqueductal grey (PAG, analgesia), substantia nigra (dopamine; damaged in Parkinson's), red nucleus, reticular formation.

Diencephalon

Contains the thalamus and hypothalamus. Thalamus: major sensory relay — receives visual, auditory, and somatosensory signals and projects them to the cortex (two-way). Hypothalamus: motivated behaviour (eating, sleeping, sex) via hormone regulation with the pituitary gland.

Telencephalon

The most rostral (highest/newest) brain division. Contains: cerebral cortex (4 lobes), limbic system (amygdala, hippocampus, fornix, cingulate, septum, mammillary bodies), and basal ganglia (striatum, globus pallidus, amygdala). Controls highest cognitive functions.

Limbic system

A functional system of subcortical structures involved in motivated behaviour — the '4 Fs': Feeding, Fighting, Fleeing, and sexual behaviour. Key structures: amygdala (emotion/fear), hippocampus (memory), mammillary bodies (memory), fornix, cingulate cortex, septum.

Basal ganglia

Subcortical structures regulating voluntary movement and decision-making. Key structures: caudate + putamen (collectively = striatum), globus pallidus, amygdala. Critical pathway: substantia nigra (midbrain) → striatum via dopamine. This pathway degenerates in Parkinson's disease.

Thalamus

Part of the diencephalon. The brain's major sensory relay centre — receives sensory information from visual, auditory, and somatosensory systems and projects it to the cortex in a two-way fashion. Also relays motor signals.

Hypothalamus

Part of the diencephalon, sitting below the thalamus. Implicated in motivated behaviour (eating, sleeping, sexual behaviour, thermoregulation) through hormone regulation via the pituitary gland. Contains the optic chiasm and mammillary bodies.

Suclus

Small grooves/furrows between gyri.

Gyrus

Raised bumps or ridges on the cortex.

Fissure

Larger, deeper groove. Longitudinal one the large groove separating the two cerebral hemispheres.

Resting Membrane potential

The electrical potential difference across a neuron's membrane when at rest: −70 mV. This is the baseline from which all neural signalling begins.

EPSP (excitatory postsynaptic potential)

A graded depolarisation of the postsynaptic membrane caused by NT binding. Makes the neuron more likely to fire an AP by moving the membrane potential toward threshold.

IPSP (inhibitory postsynaptic potential)

A graded hyperpolarisation of the postsynaptic membrane caused by NT binding. Makes the neuron less likely to fire an AP by moving the membrane potential further from threshold.

Spatial summation

Integration mechanism - simultaneous EPSPs/IPSPs arriving from different locations on the membrane add together.

Temporal summation

EPSPs/IPSPs arriving close together in time superimpose and add. Integration mechanism that allows neurons to integrate inputs to decide whether to fire.

Action Potential (AP)

An all-or-none electrical signal fired when membrane potential reaches threshold (~−65 mV). Threshold → Na⁺ rush in (depolarise to +50 mV) → Na⁺ channels close, K⁺ rush out (repolarise) → brief hyperpolarisation → return to rest.

All-or-none principle

An action potential either fires fully or not at all — there are no partial APs.

Absolute refractory period

No Action Potential possible regardless of stimulus strength — Na⁺ channels are inactivated.

Relative refractory period

Follows the absolute period. An Action Potential is possible but requires a stronger-than-normal stimulus. Refractory periods ensure APs travel in ONE direction only.

Saltatory conduction

The jumping of action potentials from node of Ranvier to node of Ranvier along myelinated axons. Much faster than continuous conduction in unmyelinated axons.

Exocytosis (NT release)

The process of neurotransmitter release at the terminal button. AP opens voltage-gated Ca²⁺ channels → Ca²⁺ enters → synaptic vesicles fuse with presynaptic membrane → NTs released into the synaptic cleft → diffuse to postsynaptic receptors.

Ionotropic receptors

NT binds → ion channel opens directly → fast, brief EPSP or IPSP.

Metabotropic receptors

NT binds → G-protein activated → either opens ion channel indirectly or triggers second messenger cascade → slower, longer-lasting effects.

Autoreceptors

A type of metabotropic receptor located on the presynaptic membrane. They bind the neuron's own released neurotransmitter and act as gatekeepers monitoring NT levels in the synapse and regulating further release.

Reuptake

The most common NT clean-up mechanism. Transporter proteins in the presynaptic membrane pull released neurotransmitters back into the presynaptic terminal for recycling into new vesicles.

Glial cells

Non-neuronal cells in the nervous system essential for insulating nerve fibers, providing structural and nutritional support, modulating synaptic communication, and regulating immune responses.

Oligodendrocytes

In the CNS: produce myelin, block regeneration.

Shwann cells

In the PNS: produce myelin, promote regeneration via neurotrophic factors and CAMs.

Gap junctions

Direct cell-to-cell connections via protein channels that bridge narrow spaces between cells. Allow passage of electrical signals and small molecules without NT release. Much faster than chemical synapses. Mostly occur between cells of the same type.

Molecule neurotransmitters

• Small NTs (e.g. glutamate, dopamine): synthesised in terminal button cytoplasm; released into directed synapses; activate ionotropic receptors → rapid, brief signals.

• Large NTs/neuropeptides (e.g. endorphins, NPY): synthesised in cell body ribosomes; diffuse release; activate metabotropic receptors → slow, long-lasting, widespread signals.

Threshold of excitation

The membrane potential level (−65 mV) at which voltage-gated Na⁺ channels open and an action potential is triggered. Neurons integrate all incoming EPSPs and IPSPs. Below = no AP (all-or-none).

Neurotransmitters

Chemical messengers that allow neurons (nerve cells) to communicate with each other, muscles, or glands.